We have contrologix PLCs working in redundancy. The PLCs are connected to Flex I/O modules through Contol Net in 2 links, i.e. 2 CN modules in each rack. Untill now we used a third rack in which two other CN modules were installed. Is it possible to eliminate the third rack ? would it have any effect on performance?

see drawing attached.

So your are not going to use the third one, why you want to remove this

I'm sorry, I do'nt understand your reply.

Can you explain how the 3rd rack was functioning?

You do not mention what version of CLGX 5000 you are using. I guess when originally set up this was 8.52 or greater, but not 15. You'll need to uae version 15 for this to work. Just like your cnet nodes are consecutive your ENET nodes will need to be consecutive as well.

I think that version 13 of ControlLogix firmware was the first to support 1756-ENBT modules in the chassis with the redundant controllers. The 1757-SRM modules manage the IP addresses when you put the modules in the controller chassis.

This system will work if you remove the third 1756 chassis, because each ControlNet segment will still have two ControlNet nodes in addition to the ControlNet modules in the controller chassis. This is required to keep the network alive during switchover.

If you left the third 1756 chassis in place but moved the 1756-ENBT modules to the controller chassis, everything would continue to work the same, but you would be certain to maintain the ControlNet link if one of the FLEX adapters was disconnected during switchover.

The ControlLogix Redundancy System User Manual is very valuable for understanding ControlLogix redundancy. It is Publication 1756-UM523.